doubt in generics........

[Ganeshkumar cheekati]

SOURCE:K&B chapter:7,p.no:634.

import java.util.*;
public class GenericTest1
{
public static void main(String[] a)
{
ArrayList<Integer> input = null;
List<Number> output = null;
output = process(input);
}
public static <E extends Number> List<? super E> process(List<E> nums){
}
}

Here we can assign ArrayList<Integer> in to process method so E is Integer here�
We are returning List<Number> �
Actual return type is List<? Super Integer>.Number is super class of Integer then
Why it is giving compilation error�?

[Ankit Garg]

The method will return <? super Integer>. Now it doesn't mean that it will return only Number. Integer's super types are Number, Serializable, Comparable and Object (through Number). So <? super Integer> could mean <Object> which cannot be stored in <Number>. This is why it gives compilation error...

[M Srilatha]

Actual return type is List<? Super Integer>.Number is super class of Integer then
Why it is giving compilation error�?

Actually the compiler cant check whether the returned one is really a List<Number>. When List<? Super Integer> is used as return type, the method can return a List of any super type of Integer. So it can be List<Number> or List<Object> or some other super type of Integer.
So you can just assign the returned value to List<Number>!

Actually you can assign the returned value only to List<?> or List<? Super Integer>!

Hope this helps!
[ November 27, 2008: Message edited by: M Srilatha ]

[Ganeshkumar cheekati]

<? super Integer> means it can allow Integer (or) any super class of an Integer..


am i right?

[M Srilatha]

I guess only Super type of Integer are allowed! But not sure.

[ November 27, 2008: Message edited by: M Srilatha ]
[ November 27, 2008: Message edited by: M Srilatha ]

[Ankit Garg]

<? super Integer> means Integer or it's super type are allowed. Also note that I have used the word type and not class as <? super Integer> means it can also accept super interface of Integer...

[Ganeshkumar cheekati]

can anybody explain this?

Given a method declared as:
public static <E extends Number> List<? super E> process(List<E> nums)
A programmer wants to use this method like this:
// INSERT DECLARATIONS HERE
output = process(input);
Which pairs of declarations could be placed at // INSERT DECLARATIONS HERE to allow
the code to compile? (Choose all that apply.)
A. ArrayList<Integer> input = null;
ArrayList<Integer> output = null;
B. ArrayList<Integer> input = null;
List<Integer> output = null;
C. ArrayList<Integer> input = null;
List<Number> output = null;
D. List<Number> input = null;
ArrayList<Integer> output = null;
E. List<Number> input = null;
List<Number> output = null;
F. List<Integer> input = null;
List<Integer> output = null;
G. None of the above.

answer is B,E,F.

[M Srilatha]

See the post Ch 7 Q 16 !

This question is explained there.

[Ankit Garg]

Ganesh the question that you have posted is wrong. I was wondering that how can B,E and F be the right answer. You posted this

public static <E extends Number> List<? super E> process(List<E> nums)

while it is

public static <E extends Number> List<E> process(List<E> nums)

[Ganeshkumar cheekati]

yes just now i have seen errata.....

thanku so much srilatha for your quote...

thanks to everyone for spending your valuable time.....